Pressing a modifier key twice should 'lock' the key so that all subsequent key presses automagically become [mod]+[key]; this has worked as far back as Hardy Heron, but breaks in Precise Pangolin.
e.g., Shift, Shift, abc123 should equal ABC!@# instead of abc123.
The xkbwatch tool from x11-xkb-utils is the best way to see this--it shows that no modifier key can ever reach the locked state. This is a huge deal for people with mobility impairments who type with one finger, a pen, or, like me, a mouthstick.
XKB is the sole reason I was able to switch from Windows to Linux; it is very critical to my daily use. (Emacs is horrible without being able to lock modifier keys.)

(In reply to comment #2)
> Hmm, have you got a custom layout with latching shift? This isn't in the
> default xkeyboard-config dataset ...
I don't have a custom layout, no. I installed the standard 12.04 and Shift never locked--it only latches. Will it lock for you?

Slashdot's Sticky Keys post made me revisit this bug report, which I noticed had been changed to Resolved/Fixed.
I quickly downloaded Ubuntu 13.10, which uses xorg-server 1.14.3, but it still fails.
Peter, would you be willing to give me instructions on setting up a development box for Xorg?
Using different distros, I have narrowed the bug to somewhere between xorg-server versions 1.10.4 and 1.11.3--possibly even 1.10.6 and 1.11.1--but I can't tell which commit introduced it.
If I knew how to set up a virtualbox image to build Xorg on (e.g., what to use as a base system and which dependencies to install), I would happily bisect this bug and patch it myself. Can you help me with that?

Thank you, Alex.
The RequiredPackages* page shows me everything I to build X on Ubuntu 11.10, which is the last version of Ubuntu without this bug. I will install 11.10 in Virtualbox and start bisecting.
* http://www.x.org/wiki/RequiredPackages/

(In reply to comment #7)
> I quickly downloaded Ubuntu 13.10, which uses xorg-server 1.14.3, but it
> still fails.
quick check shows that this patch isn't in 1.14.3 or on any other 1.14 release, sorry. Your best bet is to just take that patch, apply it to your .deb source and test it. It's going to be a lot harder setting up a whole tree.